'Please tell us how we're supposed to "design PNGs with ClearType in mind"...'
Simples. When you render the text into the bitmap (remember, we're talking about bitmaps of magazine pages -- they usually have some text in them) and you use a system that uses ClearType or an equivalent, the text will have the color-element-based anti-aliasing applied. Saving that to a PNG will preserve the color pixels but not the reason for them, so a PNG renderer which needs to scale the bitmap (say by 2x) and does not include its own intelligent raster-based antialiasing routine which can properly blend the extraneous color information, will make the text look somewhat more colorful than it should.
Incidentally, tablets will be especially susceptible to this, since, when the tablet is in native orientation, the pixels will be likely be in R G B sequence, but at 90 degrees counter-clockwise (or anticlockwise, if you like), the pixels will be in
sequence. Furthermore, at 180 degrees, the pixels will be in B G R sequence, and at 270 degrees, you'll get
order. These rotations will make any ClearType-style anti-aliasing look very wrong.
In short, as Buzzword correctly pointed out, ClearType is definitely not designed for tablets to begin with, and if it were used on the systems generating the PNGs to begin with, it would generate bitmaps which would not look good on 3/4 of a tablet's possible rotations to begin with, and which would not scale well. I'm sorry his post was too complicated for you to figure out, but that is why we have the "technical content" icon.